1. Field of the Invention
The present invention relates to a directional coupler.
2. Description of the Related Art
Recently, high-power radio-frequency transmitting apparatus are used in mobile phone base stations and industrial radio-frequency heating apparatus. The high-power radio-frequency transmitting apparatus amplify an input signal with a radio-frequency amplifier and send the amplified input signal via an antenna into air or a heating chamber.
It has been customary to position a directional coupler between a radio-frequency amplifier and an antenna to monitor the output signal of the radio-frequency amplifier for its magnitude or distortion, and adjust the gain of the radio-frequency amplifier so that it will not produce an output signal having a level higher than a standard level or adjust the input signal to the radio-frequency amplifier to remove distortions from the amplified signal.
Directional couplers disclosed in Japanese Laid-Open Patent Publication No. 2002-280812 and Japanese Laid-Open Patent Publication No. 2009-027617, for example, are known as directional couplers for monitoring the output signal of a radio-frequency amplifier.
According to the related art, there are known directional couplers having a structure which includes a coupling line in addition to a main line connected between an input terminal and an output terminal (see Japanese Laid-Open Utility Model Publication No. 05-041206, Japanese Laid-Open Patent Publication No. 10-022707, and Japanese Laid-Open Patent Publication No. 11-261313).
In mobile phone base stations or radio-frequency heating apparatus, an impedance mismatch may be developed to reflect a portion of a signal to be sent from an antenna back to a radio-frequency amplifier due to environmental changes such as weather changes around the antenna or conditions in the chamber of the radio-frequency heating apparatus. Such a reflected signal may tend to cause the radio-frequency amplifier to operate unstably or, in worst cases, to break down.
One protective countermeasure against such phenomena is to place an isolator between the radio-frequency amplifier and the antenna. The isolator protects the radio-frequency amplifier by sufficiently attenuating the signal reflected from the antenna before it reaches an output terminal of the radio-frequency amplifier. According to another solution, the radio-frequency amplifier is protected by monitoring the reflected signal and stopping the input signal from being applied to the radio-frequency amplifier or turning off the power supply of the radio-frequency amplifier without delay when some trouble is detected in the monitored signal. However, there has not been proposed yet any electronic component of simple structure which is capable of monitoring a reflected signal.
The directional couplers disclosed in Japanese Laid-Open Patent Publication No. 2002-280812 and Japanese Laid-Open Patent Publication No. 2009-027617 are designed to monitor the output signal, e.g., from a radio-frequency amplifier which is input to the directional coupler, but not to monitor the reflected signal. Similarly, the directional couplers disclosed in Japanese Laid-Open Utility Model Publication No. 05-041206, Japanese Laid-Open Patent Publication No. 10-022707, and Japanese Laid-Open Patent Publication No. 11-261313 are not designed to monitor the reflected signal though they are designed to be highly versatile for use in a plurality of frequency bands.